1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display element, a device for driving a liquid crystal display element and a liquid crystal display apparatus.
2. Description of Related Art
Usually the liquid crystal display apparatus includes a liquid crystal display element and a device for driving the liquid crystal display element.
The liquid crystal display element basically comprises a pair of substrates and a liquid crystal layer disposed between the substrates. By applying a predetermined driving voltage to the liquid crystal layer, the arrangement of liquid crystal molecules is controlled so that light incident on the liquid crystal element is modulated to perform a desired image display.
A wide variety of liquid crystal display elements have been proposed. In recent years, research has been conducted on a liquid crystal display element including a chiral nematic liquid crystal composition prepared by adding a chiral material to a nematic liquid crystal, the composition being caused to exhibit a cholesteric phase at room temperature due to the chiral material.
This type of liquid crystal display element is usable, for example, as a liquid crystal display element of reflection type utilizing a selective reflection capability of the chiral nematic liquid crystal composition.
In the reflection type liquid crystal display element, an image display can be performed by applying a high or low pulse voltage to switch the liquid crystal composition to a planar state (colored state) or to a focal conic state (transparent state).
Even after stopping the application of such pulse voltage, the liquid crystal composition can be kept in the planar state or the focal conic state, in other words, the liquid crystal composition can exhibit the so-called bi-stable property or can achieve a memory effect, whereby the image display can be kept after stopping the application of voltage.
The reflection type liquid crystal display element can perform a monochromatic (mono-color) image display utilizing a black or similar color background, a 2-color image display or a full color image display.
To realize, for example, an image display in full color, it is possible to use a laminate type liquid crystal display element including at least three liquid crystal layers, i.e. a red liquid crystal layer which can perform a red display, a green liquid crystal layer which can perform a green display, and a blue liquid crystal layer which can perform a blue display.
When at least one liquid crystal layer of this laminate type liquid crystal display element is maintained in a planar state (colored state), red, green, blue or other color can be displayed. When the laminate type liquid crystal display element is maintained in a focal conic state (transparent state), black color or like background color can be displayed.
In the liquid crystal display elements, usually electrodes are formed on the pair of substrates between which the liquid crystal layer is held, and are disposed so that the electrode-forming surfaces of the substrates are opposed to each other.
For example, a liquid crystal display element has an image display region composed of a plurality of pixels which are driven by a matrix driving system using a plurality of scanning electrodes and a plurality of signal electrodes which are opposed to each other.
In this liquid crystal display element, for example, a plurality of strips of scanning electrodes (or signal electrodes) extend in a predetermined direction with a specified gap in parallel with each other on one of the paired substrates, while a plurality of strips of signal electrodes (or scanning electrodes) extend in a predetermined direction with a specified gap in parallel with each other on the other substrate. The two groups of electrodes extend across each other when viewed from a plane. Each of the pixels corresponds to a portion of electrodes which intersect each other on the paired substrates.
Each electrode formed on the paired substrates is connected to a device for driving the liquid crystal display element. When a predetermined driving voltage is applied to the electrodes on the substrates from the driving device connected to the electrodes, the liquid crystal is driven to display a desired image.
The liquid crystal display element can be driven, for example, by a simple matrix driving method.
In the simple matrix driving method, the device for driving the liquid crystal display element includes, for example, a scanning driving IC connected to the plurality of scanning electrodes and capable of supplying a predetermined selection signal voltage to the scanning electrodes and a signal driving IC connected to the plurality of signal electrodes and capable of supplying a predetermined rewriting signal voltage to the signal electrodes.
The scanning electrodes are successively brought to a selected state by successively applying the predetermined selection signal voltage to each scanning electrode from the scanning driving IC connected to the plural scanning electrodes, while applying the predetermined rewriting signal voltage to each signal electrode in synchronization with application of the selection signal voltage to each scanning electrode from the signal driving IC connected to the plural signal electrodes to apply to the liquid crystal a voltage corresponding to a potential difference between the selection signal voltage and the rewriting signal voltage, whereby the liquid crystal is driven.
When the liquid crystal is driven by such simple matrix driving method, a voltage to be applied to the liquid crystal may be, for example, an alternating voltage which undergoes a periodical change in polarity of voltage waveform in each frame (for example, rectangular pulse voltage involving a periodical change in polarity of voltage waveform) from the viewpoint of increase in lifetime of the liquid crystal and others.
However, when an alternating voltage involving a periodical change in polarity of voltage waveform is applied to the liquid crystal which serves also as a condenser, a current more easily flows between the electrodes having the liquid crystal therebetween with an increase in the waveform repeating frequency of such alternating voltage, in which case the consumed power is increased for driving the liquid crystal display element.
Further, since a voltage corresponding to the alternating voltage is supplied to the scanning driving IC in one frame, the scanning driving IC is required to have a capability of withstanding a voltage corresponding to a difference between maximum and minimum voltages in the alternating electric field.
In the simple matrix driving method, a rewriting signal voltage is applied to the signal electrode corresponding to a pixel to be displayed in synchronization with the selection signal voltage in each scanning time in which the selection signal voltage is applied to the scanning electrode, and a voltage is applied based on the selection signal voltage and the rewriting signal voltage to the liquid crystal corresponding to the pixel to be displayed. In this operation, a voltage is applied to the liquid crystals corresponding to pixels not to be displayed by the rewriting signal voltage. Namely the so-called “cross-talk” occurs.
Due to the cross-talk in the the liquid crystal corresponding to those pixels, for example, one or more of the pixels to be displayed in high density are displayed in slightly low density, or one or more of the pixels to be displayed in low density are displayed in slightly high density. Namely an image like a shadow appears in the foregoing pixel portions. In other words, the phenomenon of shadowing occurs.